We propose to cleave the DNA of bacteriophage phi X 174 into a set of genetically defined specific fragments suitable for detailed chemical studies. DNA fragments will be produced using specific DNases such as bacterial restriction enzymes. The resulting fragments will be characterized genetically by biological assay. We propose to initiate studies of nucleotide sequences within isolated fragments of phi X 174 DNA. We propose to continue our studies of the in vitro synthesis of genetically recombinant phi X 174 DNA, by constructing a model system for "break and join" recombination. Genetically marked double-stranded phi X 174 DNA molecules will be cleaved, partially digested by an exonuclease, annealed, then repaired by polymerase and ligase. The resulting molecules will be assayed biologically for the production of genetically recombinant molecules.